The present invention relates generally to fume hoods, and more particularly to a fume hood provided with an air directing member uniquely formed to direct the inflow of air into the chamber of the fume hood in a path that prevents fumes from exiting the chamber.
Fume hoods are used in laboratories and other similar locations where it is necessary for technicians to work with materials that generate noxious and poisonous fumes. These fume hoods are formed with an interior, generally enclosed chamber in which the work is done, and the front of the chamber has an access opening through which the arms and hands of the technician can extend to work on materials within the chamber. The fume hood includes a sash or door that moves vertically to open and close the access the opening, or to vary the size of the access opening.
Because of the harmful effects of the fumes generated within the chamber of the fume hood, it is imperative that these fumes be maintained within the confines of the fume hood and then be exhausted therefrom so they will not be permitted to escape though the access opening and endanger the technician standing just outside the access opening. For this purpose, fume hoods are provided with exhaust systems of some kind that exhausts the air and fumes from the interior of the chamber and then transports them to a location outside the fume hood where they can be safely disposed of. The exhaust system creates a negative pressure within the chamber that, in turn, causes outside air to flow into the chamber through the partially open access opening between the bottom of the sash and the work surface of the chamber, and this inward flow of outside air tends to prevent the fumes from exiting the chamber through the access opening. However, if this inward flow of air is not properly directed, some of the fumes will leak out through the access opening and present an undesirable and often dangerous situation for the technician working in the fume hood.
For example, where no structure or device is provided for directing the inward flow of air into the chamber, the momentum of the inward flow of air entering a region of lower static pressure tends to cause a vortex of air to form adjacent along the bottom surface of the chamber and adjacent the front edge thereof. This vortex captures some of the fumes within its generally circular flow pattern, and the vortex is very sensitive to any variation of air pressure or airflow. As a result, even movement of the technician, or someone walking past the partially open access opening can create an external pattern of airflow that is sufficient to disrupt the vortex to an extend that fumes will be released from the chamber through the access opening.
One of the first attempts to correct this problem was to add a permanent air directing plate, or airfoil, that was mounted to the fume hood to extend into the chamber through the bottom of the access opening at a location about one inch above the bottom wall of the chamber. The airfoil is shaped to direct the incoming air along the bottom surface of the chamber, beneath the airfoil, and this airflow essentially eliminates the undesirable fume-carrying vortex. However, this airfoil has its own disadvantages. First, because it is a permanent structure extending through the access opening and into the chamber just above the bottom wall thereof, the airfoil creates a stop member for the sash in its downward movement that prevents the sash from fully closing. Therefore outside air constantly flows into the chamber beneath the airfoil which increases the energy costs in operating the fume hood and its exhaust system. Additionally, because of its location, it creates an obstacle when objects and equipment, particularly heavy objects, had to be moved into and out of the chamber through the access opening.
Another solution to the vortex problem is to mount a simple flat air-directing member at the front edge of the bottom wall of the chamber at an incline thereto and outside of the path of vertical movement of the sash. Because of its location, this airfoil does not interfere with the movement of the sash and does not form a barrier to the movement of objects into and out of the chamber. This type of airfoil has been found, in most cases, to effectively eliminate the formation of the undesirable vortex by directing the incoming air along the surface of the bottom wall of the chamber. However, this type of airfoil is effectively only when the velocity of the airflow into the chamber is at or above approximately seventy feet per minute (70 FPM). Energy conservation considerations now make it very desirable to use a smaller-capacity exhaust system, and reduce the airflow velocity into the chamber to something less that 70 FPM, and at this reduced velocity the air flow directed by the airfoil tends to separate from the bottom surface of the chamber and allow “dead” air containing fumes to form along the bottom surface of the chamber and beneath the separated airflow created by the airfoil. This dead air, like a vortex, is sensitive enough that even small disturbance of the air outside of the access opening (e.g. a technician walking by the front of the fume hood) can cause the dead air with the fumes entrained therein to be drawn outwardly though the access opening.
It has also been proposed to mount an airfoil at the front end of the fume hood for pivotal movement between an operable position and a position where the airfoil has not effect on the airflow. In U.S. Pat. No. 5,556,331 there is disclosed a fume hood with a pivoted airfoil having a single air-directing surface that, in its operable position, directs air along a path parallel to the bottom surface of the fume hood. It appears this construction would have the same problem described above where the velocity of the air is less the 70 FPM. U.S. Pat. No. 6,582,292 discloses another pivoted airfoil having multiple air-directing surfaces. In both of these patents, the air directing member is positioned inwardly of the plane of the sash or door, and this location creates problems in terms of maintaining the inward flow of air along the bottom surface of the interior of the fume hood, particularly at low air velocities.